The present invention relates to a photosensor used in a storage mode, and more particularly to a photosensor which is used in a photoconductive imaging tube or solid-state imaging device and has a photoconductive layer made of hydrogen-containing amorphous silicon.
The hydrogen-containing amorphous silicon is photoconductive, and moreover a large-area, thin and uniform film thereof can be formed at lower deposition temperatures as compared with such a thin film of monocrystalline silicon. Therefore, it has been proposed to make the photosensing area of a solar cell, imaging device or the like of the hydrogen-containing amorphous silicon. A photoconductive imaging tube shown in FIG. 1 is a typical example of photosensors used in a storage mode, and includes a transparent substrate 1 (usually called a face plate), a transparent conductive layer 2 or a transparent electrode, a photoconductive layer 3, an electron gun 4, and an envelope 5 (for example, a glass tube). Light having passed through the face plate 1 is focussed on the photoconductive layer 3 to form a light signal, which produces photo carriers in the photoconductive layer 3. The photo carriers thus produced are stored in a surface of the layer 3 in the form of a charge pattern, and are successively read out. In general, a beam landing layer 7 is provided on another surface of the photoconductive layer 3, to prevent the secondary electron emission from the layer 3 and to prevent an electron from being injected from a scanning electron beam into the photoconductive layer 3.
The present inventors have proposed to make the above-mentioned photoconductive layer of a hydrogen-containing amorphous material whose main constituent element is silicon. This proposal is described in U.S. Pat. No. 4,255,686 and U.S. patent application Ser. No. 257,611 now U.S. Pat. No. 4,419,604.
An image pickup tube having a photoconductive layer of amorphous silicon is high in photoelectric conversion efficiency, and is excellent in resolution and heat resistance. In a conventional image pickup tube of this kind, the photoconductive layer 3 is formed of an amorphous silicon film having a high electric resistance, that is, a resistivity of not less than 10.sup.8 .psi.cm. In more detail, the electric resistance of the film is made high to cause the image pickup tube to perform a storage type operation, and to increase the resolution of the tube. The high-resistance film has hitherto been formed in two manners. That is, a film showing a special infrared absorption peak is formed by a reactive sputtering method, or a very small amount of impurities are added to a deposited amorphous silicon layer by a glow discharge method, as described in a Japanese Patent Application Laid-open No. 153782/81. In the case where the glow discharge method is used, an amorphous silicon layer which is doped with arsenic, phosphorus or the like at a concentration of about 200 ppm to have a resistivity of not more than 10.sup.6 .psi.cm, can be interposed, as a blocking layer, between a transparent electrode and a high-resistance amorphous silicon layer having a resistivity of not less than 10.sup.8 .psi.cm and serving as a photoconductive layer, to improve the characteristics of an image pickup tube. In either method, however, the whole of a photoconductive layer which is a main part of a photoelectric conversion section, is formed of a single film having a uniform composition. Accordingly, it has been observed that the sensitivity and lag characteristics of an image pickup tube vary with the frequency of signal light.
In more detail, in the case where a picture image is formed of blue light, a voltage necessary to sufficiently take out a signal from the photoconductive layer is higher, as compared with the case where the picture image is formed of red light. And further, the decay of afterimage depends upon the frequency of signal light. These are very serious problems when an amorphous silicon layer is used in an image pickup tube for color TV.